1. Field of the Invention
This invention generally relates to telecommunications networks and more specifically to network elements in ring networks.
2. Description of the Related Art
The arrangement of network elements in a telecommunications network is known as “topology”. In Synchronous Optical Network (SONET), for example, network elements can be arranged in a ring or a linear topology. Network elements in a linear topology are arranged along a line, whereas in a ring topology the network elements are arranged in a circular fashion.
SONET is well known and described in the following documents: American National Standards Institute (“ANSI”) documents ANSI T1.105, ANSI T1.105.01, ANSI T1.105.02, ANSI T1.105.03, ANSI T1.105.04, ANSI T1.105.05, ANSI T1.105.06, ANSI T1.105.07, ANSI T1.105.08, and ANSI T1.105.09; Bellcore Standards GR-253-CORE (Issue 2, December 1995), GR-1230-CORE (Issue 4, December 1998), GR-1375-ILR (Issue 1A Revision 1, August 1995), GR-1400-CORE (Issue 1, March 1994, Revision 1, October 1995), and TR-NWT-000496 (Issue 3, May 1992); see also, W. J. Goralski, “SONET: A guide to Synchronous Optical Networks,” McGraw-Hill 1997. All of the aforementioned SONET documents are incorporated herein by reference in their entirety.
SONET specifications provide for a number of self-healing optical ring topologies including the Unidirectional Path Switched Ring (UPSR) and the Bidirectional Line Switched Ring (BLSR), both of which are well known. In a UPSR ring, the originating network element transmits duplicate SONET frames on two communications links. The receiving network element receives the frames from both links and, depending on the quality of the received signals representing the frames, uses the frame from one of the links. The receiving network element does not have to notify the transmitting network element if one of the links is locally detected to be defective.
In a BLSR ring, the SONET frames are transmitted by the originating network element on a working link. When the receiving network element detects that the working link is defective, it so informs the transmitting network element and initiates a switchover to a protect (i.e. back up) link. Coordination between network elements in switching to a protect link is performed using a signaling protocol which uses overhead bytes of the SONET frames.
It is desirable to have a single network element that can support multiple rings. The flexibility afforded by such a network element reduces the cost of the network and simplifies the interconnection of rings.